1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling vehicle motion, and more particularly to the control system for maintaining vehicle stability by controlling the braking force applied to each wheel of the vehicle.
2. Description of the Prior Art
Recently, a vehicle is provided with a braking force control system for controlling the braking force applied to the vehicle to perform an anti-skid control, a traction control, a front-rear braking force distribution control, and etc. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering.
According to the prior apparatus as described above, both of an increasing rate (or gradient) and a decreasing rate of the braking pressure in a wheel brake cylinder operatively connected to a wheel under control are constant. Therefore, such a problem may be caused that the vehicle motion will be varied rapidly or gradually, depending upon a coefficient of friction of a road surface or a condition of the vehicle in motion, when a vehicle stability control starts or ends. Generally, it is desired that a smooth shift from a braking pressure generated by a master cylinder in response to depression of a brake pedal by a vehicle driver, i.e., a master cylinder pressure to a braking pressure for controlling the vehicle stability, i.e., a control pressure, so as to provide a smooth initiation of the control, and its reverse shift, i.e., a smooth termination of the control shall be made. Especially, when the vehicle stability control is initiated, it is desired that the rate for shifting the master cylinder pressure into the control pressure shall be controlled in response to the coefficient of friction of the road surface, thereby to ensure an appropriate follow-up control.
Furthermore, when the vehicle stability control operation is terminated, it is important to operate the braking force control system so as to shift its output braking force smoothly into the one equal to the braking force generated by the master cylinder. If the braking force shift is not made smoothly, a pedal shock will be caused to the vehicle driver. For example, in the case where the vehicle stability control is made by the braking pressure control so as to apply a relatively large braking force to a wheel located at the front-right side of the vehicle during the vehicle stability control operation, and apply a relatively small braking force to a wheel located at the front-left side of the vehicle, if the vehicle stability control is terminated, the braking pressure applied to the front-right wheel is reduced rapidly into the braking pressure discharged from the master cylinder in response to depression of the brake pedal. Therefore, the pedal shock will be caused, and the vehicle stability control might be deteriorated. Likewise, when the vehicle stability control is initiated, the pedal shock may be caused by a rapid shift from the master cylinder pressure to the control pressure, so that the driver's feeling may be deteriorated.